2 reasons, surface area and direct heat.
Surface area: If you step your foot in a puddle, the puddle touches more of your foot than the ground would if it was dry. The puddle conforms to your foot and can seep into the cracks, crevices and pores of your foot. That’s what oil does to a piece of food in a pan. The more hot oil touches your food, the more seared your food will be.
Direct heat: any piece of food in a frying pan is either pressed flat to the pan and receiving direct heat, or it’s slightly lifted from the pan and receiving indirect heat (either baking it or steaming it). Heating oil allows the direct heat to come from the pan *and* the oil. Set a piece of broccoli in hot oil, it’s getting cooked, give it a turn and the hot oil is still cooking it on the side that’s not touching the pan.
Heat transfer due to increased surface area contact. Most food surfaces are not flat, but the cooking surface usually is. This means the food is only in direct contact with the heat in those spots that can actually touch the cooking surface. The hot fat/oil helps transfer the heat of the cooking surface to the uneven food surface, allowing for a more even looking sear.
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